![]() ![]() Overlay with fiberglass completely solves the checking.įor my race boats I use Okume plywood (3mm, 6mm, etc.) and AB exterior for transom boards laminated with Okume for a bright finish, clear coat with epoxy + fumed silica in epoxy for a structural thickener for bonding. The AB never failed the test.įir plywood will have some grain checking and needs good sealing to temper that, the polyester resin did a good job doing this in our outside applications. If using solid wood, the same applies, 1. Transoms are typically reinforced with knees, usually a couple at the rail and two or three on the bilge stringers, to transmit torsion loads to the bottom. Most use two layers of 3/4' ply, but a stronger method is three layers of 1/2'. Then and now before using any plywood a WBP test was done on a small piece. Outboard transoms are pretty much standardized at 1.5'. Metals (SS, Monel, marine Aluminum) were also used in special cases along with the plywood. We used the plywood because it was quick to build and to modify as conditions arose and relatively inexpensive. Fiberglass cloth was used in some areas where extra joint strength was needed. There never was a failure and they held up excellent for exterior and interior use with sea water. I specified AB Fir exterior plywood for these and to use 2 part epoxy and 3 coats of polyester resin. When I was a PM engineer at a Marine Research facility in West Palm Beach, Fl in the 70’s we built numerous sea water use large and small tanks, flumes, large pump forbay sumps, transport tanks for long haul on land and air, etc. There are ply thickness standards used, Ex: I have seen 5 ply 3/4" and 7 ply 3/4". I have built boats with marine and HQE plywood with equal success and HQE costs less. ![]() However you might want to consider a solution that one of my sailing mates constructed for his Welsford Pathfinder. High quality exterior (HQE) plywood like AB exterior grade uses the same glue as marine plywood. Your solution with a stainless stell plate sounds good but expensive but would the clamps holding the motor might not have enought 'bite' on the stainless either site, probably that is why wood is the go-to for the transom. Plywood glue specs are WBP(water and boil proof for marine and high quality exterior grades) and MR (moisture resistant usually for interior grades). ![]()
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